1. Field of the Invention
The present invention relates to a circuit board to be precoated with solder layers for allowing electronic components such as LSIs, which are provided with a large number of leads, to be mounted on the circuit board, and also relates to a solder precoated circuit board.
2. Description of the Related Art
When electronic components, the lead pitch of which is 0.5 mm or greater, are mounted on a printed circuit board, a method using solder paste is generally employed. In this method, each electronic component is mounted on the printed circuit board in which solder paste is printed on the pads thereof by means of a mounter; with the electronic component being positioned such that the leads are arranged on the pads. The printed circuit board, on which the electronic components have been mounted, is heated in a fellow furnace such that the solder paste is fused, thus soldering the leads and the pads to each other. Since the solder paste is adhesive, there is no risk of the electronic components being displaced during the processes which follow the mounting of the electric components.
Currently, electronic components such as LSIs have a tendency toward an increase in the number of leads. In contrast, there is a great request for reducing the sizes of the electronic components as much as possible in order to reduce the sizes of devices. Taking the above circumstances into consideration, the lead pitch of the electronic components is now being reduced. However, according to a conventional method utilizing the solder paste, when the lead pitch is reduced to the order of 0.3 mm, a large number of solder bridges are undesirably formed between the leads.
In consideration of the above-described drawback of the conventional method, solder precoating methods which do not use the solder paste such as the following two methods have been proposed:
(1) a method wherein solder is deposited on the pads by means of electrolytic plating or electroless plating, after which fusing (forming an alloy by fusing a plated layer) is performed; and
(2) a method wherein a solder precipitating composition, which contains tin powder and organic acid lead salt as the main components, is coated, and a substitution reaction is caused between the tin powder and the organic acid lead salt so as to precipitate solder substantially only on the pads (U.S. Pat. No. 5,145,532).
In the case of the above methods, solder layers are formed on the pads before mounting of the electronic components. After flux is applied on the solder layers, the electronic components are mounted on the circuit board, and then heat is applied to the solder layers, thus mounting the electronic components.
Depending on the accuracy with which circuit patterns are formed, it undesirably happens that pads having different sizes are formed on the printed circuit board. When a predetermined amount of solder is precoated on the pads having different sizes, thick solder layers are formed on narrow pads, and thin solder layers are formed on wide pads.
In consideration of the above, where the pad width is great, the solder precoating conditions are determined so that solder layers having a thickness necessary for soldering can be formed. The solder precoating process is conducted under determined conditions. In this case, however, the solder is provided in surplus onto narrow pads resulting in solder blisters being formed partially on the pads.
Thus, according to the conventional precoating methods described above, it is difficult to form solder layers having uniform thickness on the pads.
If a component lead is mounted on a solder layer which is not uniform in thickness, that part of the lead which is placed on a thick portion of the solder layer will deform, with the result that an appropriate fillet cannot be formed in a later soldering process.
Also, when repairing fine-pitch component leads, it is very difficult to again provide solder onto a pad from which a component lead has been detached. Therefore it is preferred that mounting of components be performed again using the solder left on the pad. In order to facilitate this, a large amount of solder has to be precoated on the pad. However, when a large amount of solder is precoated, variation in the thickness of each solder layer is considerable, aggravating the above-described drawback.